Human cytomegalovirus (HCMV) is a major cause of birth defects and opportunistic infections in immunosuppressed individuals, and a possible cofactor in certain cancers. Organ transplant patients under immunosuppressive therapy are at high risk for viral infections; activation of a latent virus as well as donor or community acquired primary infections can cause significant complications including graft rejection, morbidity, and mortality. Herpesviruses (e.g. HCMV, HSV-1), polyomaviruses (e.g. BKV and JCV), hepatitis viruses (HBV and HCV) and respiratory viruses (e.g. influenza A, adenovirus) are the 4 major viral classes infecting these patients. Cytomegalovirus (HCMV) is the most prevalent post-transplant pathogen; HCMV can infect most organs, and despite the availability of HCMV antivirals such as ganciclovir, nephrotoxic side effects and increasing rates of drug-resistance significantly reduce graft and patient survival. In addition, HCMV-mediated immune modulation can reactivate distinct latent viruses carried by most adults.